The problem of achieving a satisfactory flow of particles out of bins, hoppers, silos and all kinds of holding or retaining vessels has been the subject of studies for over a hundred years. Often, when the volume of particles to be handled is large, gravity is relied upon to cause particles to flow out of storage. Although time and money have been spent with varying degrees of success to develop containing vessels for such materials, the problem of whether or not a given solid will flow out of a given container, once it is actually built, still persists.
Whenever a container is designed to have either a mass flow or a funnel flow, numerous factors have to be considered, particularly when test results or experience show that the material to be handled tends to adhere, cake, arch, interlock or solidify over time. The designer of an efficient storage container must be aware of the problems that can arise both during the storage and during the flow of the solids to be handled. Consequently, the flow properties of the solid to be handled have to be measured to design a suitable container. It is known that the behavior of particulate solids having different flow characteristics is very difficult to predict and many problems arise when such particles are handled within a confining vessel. When such flow properties change, due to e.g. changes in temperature, moisture content, etc. provisions have to be made to compensate for such changes in the container structure. Consequently such variations in the flow properties may make the solids flow both complex and critical. An improperly made container will tend to develop a number of unfavorable bulk solids characteristics which impede the flow of particles.
The principal known causes of flow interruptions or stoppages are packing, bridging and rat-holing phenomena. The origins of such phenomena are not well known or defined. Packing is an inevitable result of a large amount of particles pressing down toward the outlet or outlets of the handling vessel. Bridging or arching occurs when the particles are interlocked and packed by the pressure head from above, forming an arch strong enough to support the entire load of the material in the vessel. Rat-holing occurs when a small cylindrical volume of the material flows down to the outlet, leaving the main body of the material hung up on the wall of the handling vessel.
There are several general approaches employed by those skilled in the art when studying the flowability of particulate solids. These involve the determination of certain parameters of flowability by subjecting a sample of the particles to a shearing action, but prediction of the particle behavior is not always accurate or complete.
Numerous solutions have been proposed and are known from the technical literature. These solutions fall mainly into two classes. First, there are those that relate to the structure of the container itself and that aim to promote a mass flow, a funnel flow or a combined flow by modifying the physical characteristics of the container e.g. the type of wall, its shape, the material of which it is made, the use of internal supports and the nature of its inlets and outlets. The second class of proposed solutions relate to auxiliary devices or methods for promoting material flow. These may be internal or external and may be e.g. mechanical vibrators attached to the container wall, internal slippery liners, agitators, injection of gases to fluidize or otherwise facilitate particle flow, as well as chemicals to aid in solving specific problems.
It has been proposed in the past in order to solve the flow problems in bins and other like vessels to make such containers with very steep wall angles, as well as to avoid any flow obstruction or irregularity in the walls so that the smooth surface prevents stoppages and in some cases to use also some kind of flow aid or promoter.